Smallpox (variola major) assumes a special place among infectious diseases in view of its enormous impact on humanity and because the disease has been eradicated throughout the world. Infections with related viruses, however, still occur. The family of Poxviridae consists of a collection of large, double-stranded DNA viruses. Recombinant pox viruses have been studied for their potential use in molecular biology and in vaccine research. Four different strains of pox viruses have been isolated in monkeys and rodents that differ from monkeypox but are genetically closely related to smallpox. Confusingly enough, these viruses are referred to as "white pox". Their significance for human infections is not yet known. In addition to a number of viruses that infect individual animal species (swinepox, rabbitpox, fowlpox, etc), the pox viruses include the following genera which are important for humans:

• Orthopox virus     : smallpox, alastrim, vaccinia, cowpox, monkeypox, camelpox
• Parapox virus     : orf, pseudocowpox (syn. pseudovaccinia or milker's nodules), sealpox, bovine papular stomatitis
• Molluscipox virus: molluscum contagiosum
• Yatapox virus     : tanapox, Yabapox

Pox viruses can infect many species of birds, and each species of bird may have its own unique species of pox virus (mynah birdpox virus, canarypox virus, etc.). Canarypox virus is being studied at present as a possible vaccine vector (e.g. AIDS vaccine). Volepox, skunkpox, raccoonpox viruses do not cause disease in humans.